Clay bleaching



United States Patent 3,290,161 CLAY BLEACHING Fred R. Sheldon,Princeton, and William H. Kibbel, Jr., and James E. Kressbach,Pennington, N.J., assignors to FMQ Corporation, New York, N.Y., acorporation of Delaware No Drawing. Filed Oct. 4, 1963, Ser. No. 313,763Claims. (Cl.'106--72) This invention relates to the bleaching andrefining of clays, and more particularly to a method of treating clayswhich increases their brightness and retains their desirable physicalproperties.

Clays, particularly refined kaolins, .are employed in a variety of usessuch as fillers and finishing agents for papers and paper-boards, asbase ingredients and as fillers for the molding of electrical and otherceramic or plastic products, and as finishing agents for a variety ofarticles. To be suitable for these applications, it is important that aclay be highly reflective, that is, that it have a high degree ofbrightness as measured on the basis of percent of light reflectance. Itis also important that the clay have a low and uniform shear viscosityto permit optimum and reproducible processing in utilization; theelectrical charges on the clay particles affect viscosity, and must beretained to as great an extent as possible.

Since the clays as they are obtained from natural sources normally .arediscolored, it has been necessary to brighten them with bleaches, andthe large volume of use of clays has been a great incentive to chemistswho have expended considerable research effort in developing bleachingmethods for them. Thus, the oxidizing bleaches, for example chlorinebleaching agents such as chlorine dioxide, hypochlorites and the like,and active oxygen bleaches such as hydrogen peroxide, sodium peroxideand the organic peroxides, have been applied to this problem, as havethe reducing bleaches such as the oxalates and the low oxidation statesulfur compounds.

Quite surprisingly, these efforts have not provided a bleachingtechinque which provides as high a degree of brightness in clays as isdesired and at the same time retains the desirable physical propertiespossessed by them. While it has been found that the reducing bleachesgenerally are superior to the oxidizing bleaches in this tip plication,even the most desirable of these agents has not provided the looked-forcharacteristics in the final bleached clay. Furthermore, processing withthese agents has been time-consuming and expensive.

It is a feature of this invention to provide a method of bleaching clayswhich 'is adapted to low-cost continuous processing and also provides adegree of brightness in the clays which is s'atisfiactory to papermakers, molders of ceramic products, producers of finishing agents andthe like.

It has now been found possible tobleach clay to a degree of brightnessunexc'elled by any prior method, by a new process which despite itseffectiveness in bleaching does not substantially degrade the clay.These desirable ends are accomplished by providing an aqueous slurrycontaining a clay in the amount of about 5 to 75%, and preferably about15 to 35%, adding to this slurry an alkali bisulfite in an amount toprovide 0.710 5.6, and preferably 2 to 4.5, pounds of the bisulfite perton of the clay, adding an alkali borohydride to this slurry in anamount to provide 0.125 to l, and preferably about 0.36 to 0.82, poundof the borohydride per ton of the clay, stirring the slurry mildly for atime to effect bleaching of the clay and thereafter washing the clay tofree it of residual bleaching reagents and reaction products. Theprocessing is carried out at a pH of about 2.5 to 5, and preferablyabout 3 to 3.5. Where necessary mineral acids such as sulfuric acid orother acids are employed 320,161 Patented Dec. 6, 1966 to provide thispH. The operating temperature used is between about 40 and 200 F., andpreferably between 70 and 180 F.

In our process the bisulfite is used in the amount of about 5.6 timesthe amount of borohydride, within the above re-cited ranges. Use ofsubstantially more than this amount does not improve bleaching, and useof less is apt to provide less bleaching than normally could be obtainedwith the given amount of borohydride.

This process provides a bleached clay having a higher degree ofbrightness than isprovided by any other method known to us. Theexcellence of this bleach is quite unexpected; when the borohydride isemployed in herein amounts alone in an attempt to bleach clay,substantially no brightness increase is obtained, and in some cases thebrightness is reduced. This is despite the fact that sodium borohydrideis known as a reducing bleaching agent'for some materials. Likewise, thebisulfite alone provides no brightness improvement.

Furthermore, although it may be theorized that sodium borohydride andsodium bisulfite might react to form the known clay-bleaching agentsodium hydrosulfite, direct addition of sodium hydrosuliite to the clayas a bleach provides a noticeably lower degree of bleaching than isprovided by the present process, even when substantially more of thehydrosulfite is employed than theoretically would be formed by reactionof the herein amounts of the borohydride and bisulfite. In fact,attempts to achieve the extent of clay brightening obtained by theherein process by addition of increasing amounts of hydrosulfite,actually reduce the brightness and result in an undesirable degradationof the clay. Furthermore, quite contrary to What would be expected ifthe bleaching mechanism were simply through hydrosulfite formation, theprocess of this invention achieves its bleaching effect more rapidlythan does hydrosulfite. These factors are not susceptible to simpleexplanation.

The clay bleached by the herein process is a hydrated alumina silicate,preferably kaolin. Other forms of hydrated alumina silicate clays, forexample the bentonites, the montmorillonites and the like, can betreated by the present process, and such clays are intended to beincluded with-in the scope of this invention.

Normally the clay is prepared for bleaching by a wash, which is carriedout by dispersing the clay in water, for example with the aid of causticor a phosphate such as sodium hexametaphosphate, and washing it toremove siliceous impurities such as sand, mica and the like. This is astandard treatment employed in clay bleaching to reduce the amount ofbleach needed and to cleanse the clay. The washed clay has a colordepending on its grade, and this color will range anywhere from a bluegray to tan up to a near white.

As the first step in the present process, the clay is slurried in waterto provide a slurry having a clay concentration of about 5 to andpreferably about 15 to 35%, by weight of clay in the water. This slurryis then treated by addition to it of an alkali bisulfite in an amount toprovide about 0.7 to 5.6, and preferably 2 to 4.5, pounds of thebisulfite per'ton of the clay. The bisulfite can be introduced in anyphysical form, although it is preferred to introduce it dry. When themetabis-ulfiteNa S O is used it converts in aqueous solution by additionof water to the normal bisulfite, NaHSO Thus, either of these bisulfitesmay be employed. It is important only that the bisulfite be in solutionin the aqueous phase of the clay slurry, and it may be introduced insolid form, in slurry form or in solution, so long'as it dissolves inthe aqueous system. Likewise, additional agents useful in dispersing theclay, for example phosphates commonly used in clay processing, may beemployed.

The bisulfites found to be particularly useful in this process are thealkali metal, alkaline earth metal and ammonium bisulfites, particularlythe sodium, potassium, lithium, magnesium and ammonium bisulfites, andmore particularly the alkali metal bisulfites. The bisulfites may haveassociated with them substantial amounts of sulfites of ammonia or oneof these metals without detriment; at the pH of the present solutionsthe sulfite converts to bisulfite.

The alkali borohydride is a solid, and normally is introduced in thisform, since in aqueous solutions it tends to decompose. It is possible,however, to introduce this reagent in its aqueous solution provided thepH of the solution is maintained slightly alkaline prior to use byaddition of an inert alkaline material such as sodium hydroxide, sodiumcarbonate and the like. Preferred alkali borohydrides for use herein arethe alkali metal borohydrides, particularly the sodium and potassiumborohydrides, although any water soluble alkaline borohydride, forexample magnesium and the like borohydrides can be used.

The alkali borohydride is employed in the amount of about 0.125 to 1,and preferably about 0.36 to 0.82, pound per ton of clay. Normally notmore than about one part by weight of the borohydride is used for each5.6 parts by weight of the bisulfite. It is important to note that useof these amounts of borohydride, or even more, in the absence of thebisulfite is ineffective to accomplish eifective bleaching of clays.

The temperature employed in the course of this process ranges from aboutambient temperatures to about 200 F., and preferably from about 70 to180 F. When elevated temperatures within this range are employed, thebleaching and refining operation proceeds rapidly, so that it isespecially suited to continuous operation. Operation at lowertemperatures within the above range is particularly useful, however, ininstances where it is not desired to install heating equipment or forother operating reasons. Operation above about 200 F. tends to decomposethe active reagents in the system and therefore is undesirableeconomically. On the other hand, one of the principal advantages of theherein process is its ability to bleach and refine clay at temperaturesencountered in water supplies even during the cold seasons. Thus it ispossible to bleach clay by this process even with water as cold as about40 F., water temperatures encountered in some areas.

The pH of the reaction system is maintained at about 2.5 to 5, andpreferably at about 3 to 3.5. Operation below about pH 2.5 results indissolution of substantial amounts of aluminum ion from the clay,whereas when the reaction is carried out above about pH 5 the brightnessof the bleached clay is reduced. Where necessary, mineral acids,preferably sulfuric acid, are added to adjust the pH to the hereinlevels.

The concentration of the clay slurry during processing is not critical.It is desirable, however, to have as high a concentration of clay andbleaching agents as is possible consistent with eflicient and uniformcontact of reactants, and to this end concentrations of from 5 to 70%,and preferably to 35%, by weight of clay are preferred.

The process of this invention operates in either a continuous or a batchfashion, in either case rapidly. This speed of the reaction may be thereason for the excellent retention of physical properties of the clay,for example retention of its rheological properties. It has been foundthat overall the process will operate in about 30 minutes at 70 F., andin only about minutes at 120 F., whereas the operation with priorprocesses often takes on the order of twice as long for full bleaching.

The various steps of the process are carried out with mild agitation inorder to assure intimate contact of the reagents with each other andwith the clay. The washing of the treated clay normally is carried outwith water, although common aids such as soaps, detergents and the likemay be added, and other washing media, for example alcohols, may beused. Separation of the clay from the Temperature, F.:

washing liquid is achieved by common means such as filtration,centrifugation, pressing and the like. The clay following sep-arationfrom the aqueous or other washing may be dried, if desired, at atemperature and pressure which will remove essentially all of the water.In some cases it is desired to use the clay as a slurry, and in suchcases the clay will not be dried.

The following examples are presented byway of illustration only, and arenot intended to limit the scope of the present process in any way.

Example 1.Preparation of clay samples for bleaching Orude kaolin asobtained from a mine was dispersed in water containing about 0.1% byweight of sodium hexametaphosphate to provide an aqueous slurrycontaining about 30% by weight of the crude clay. This slurry wasstirred in a high speed agitator for 30 minutes, following which it wastransferred to a large beaker, the slurry was diluted to provide a 20%solids slurry, and the slurry was allowed to settle for 20 minutes. Thesuspension which formed was decanted and saved, while the settledmaterial was discarded. The suspension was then allowed to settle for 48hours, and the precipitated or settled material therefrom was retainedfor bleaching studies. This material was dried overnight at 100 C. toprovide a standard sample of so-called classified clay for use in thefollowing experiments.

This classified kaolin clay had a percent reflectance brightness of83.1% at 458 microns on the GE brightness scale. The instrument employedfor this measurement was a Bausch and Lomb 505 SpectronicSpectrophotometer; brightness values presented hereinafter were alldetermined on this instrument.

Example 2 One hundred grams of the classified clay from Example 1 wasslurried in 400 g. of water with mild stirring. The pH of the slurry wasadjusted to 3.3 with sulfuric acid, and the temperature of the slurrywas raised to 120 F. This pH and temperature were maintained throughoutthe experiment. 0.138 g. of sodium metabisulfite (2.76 pounds per ton ofclay) was added to the slurry and dissolved with stirring in a fewseconds. 0.025 g. of sodium borohydride (0.50 pound per ton of clay) wasthen added to the slurry, and the resulting slurry of clay in a solutionof the metabisulfite and borohydride was mildly stirred for 30 minutes.The clay was then separated from the bulk of the bleaching solution on afilter and washed with water at room temperature, after which it wasdried at C. in an oven.

The brightness of the clay prior to the bleaching operation was 83.1%,whereas following bleaching it was 87.9%. This improvement was verynoticeable; the naked eye can detect a difference of 0.1% in brightness,and a 0.5% difference is significant in the clay trade. Likewise, theclay following the bleaching operation had a uniform and low shearviscosity making it highly suitable for use as a paper filler andfinishing agent, and as a molding ingredient in the preparation ofelectrical and other ceramic and plastic products.

Example 3 The procedure described in Example 2 was carried out in aseries of experiments in which the temperature of the bleach was variedbetween 70 and 180 F. The results of these experiments are'shown inTable I which follows.

TABLE I Brightness-percent GE reflectance (unbleached clay 83.1%)

As demonstrated by this example, the brightness of the clay is dependentupon the temperature at which the bleaching operating is carried out,with optimum brightness being achieved at about 120 F. and thebrightness dropping off slightly on either side of this temperature.Operation at 40 F. likewise bleaches the clay to a higher brightnessthan it was prior to bleaching, as does bleaching at temperatures of200F. However, the preferred range of bleaching is about 70 to 180 F.

Example 4 The procedure of Example 2 was carried out employing varyingconcentrations of sodium bisulfite and sodium As demonstrated by thisexample the amounts of bisulfite and borohydride employed can be variedconsiderably without destroying the ability of the system to bleach clayeffectively.

Example 5.Cmparative example, reducing bleach, not

within this invention A comparative bleaching operation was carried outwith sodium hydrosulfite serving as the bleaching agent. It was employedunder the conditions of Example 2 above, with the exception that inplace of the sodium metabisulfite and sodium borohydride employedtherein, 0.136 g. (2.7 pounds per ton) of sodium hydrosulfite wasdissolved in the bleaching liquor. The bleached clay provided by thisoperation had a. brightness 1.5% below that of the bleached clay fromExample 2; the clay of this Example 5 had a brightness of 86.3%. Thisdifference was very noticeable, and was considered by a claymanufacturer to be important in the production of clay for paperfinishing and filling agents. Extension of the bleaching time withsodium hydrosulfite as the bleaching agent did not result in anyincrease in brightness,rather resulting in a drop oil in brightness andadversely aifecting the viscosity of the clay. Likewise, eitherincreasing or decreasing the amount of hydrosulfite resulted in a dropoff in brightness rather than in an increase in this desirable property.

Example 6 Substitution of potassium borohydride, lithium borohydride,and magnesium borohydride for the sodium borohydride employed in Example2 results in the production of bleached clay likewise having excellentbrightnesses. The same result is achieved when the sodium metabisulfiteand sodium bisulfite of the above examples is replaced with any of thepotassium, magnesium or ammonium bisulfites.

It is evident from the above examples that the process of this inventionoperates in a short time to provide an excellent brightening of clay.Furthermore, it operates with a readily observable advantage in degreeof brightening over a comparative method employing sodium hydrosulfiteas the bleaching agent. The improvement in brightness provided by theprocess of this invention is i ighly significant to clay producers andusers; it represents an improvement readily discernible to the eye, andone which is in the area of the best bleach results obtainableheretofore.

This combination of characteristics is of great value to the clayindustry, since it makes possible upgrading of clay to an extent whichmakes previously unsatisfactory clays useable in many applications. Thisis of particular importance since deposits of high quality clays aregradually being depleted, and it is becoming increasingly important toemploy clays of a lower quality.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a. manner so thatit can be readily practice-d by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure, otherwise than as specifically described and exemplifiedherein.

We claim:

1. Method of brightening clay in which the clay is slurried in anaqueous medium, an alkali bisulfite is added to the slurry with stirringin the amount of about 0.7 to 5.6 pounds per ton of said clay, an alkaliborohydride is added to the resulting slurry in the amount of 0.125 to 1pound per ton of said clay, the temperature of the reaction system beingheld at about 40 to 200 F., and the pH being maintained at 2.5 to 5, andthe clay is washed free of residual bleaching agents and reactionproducts.

2. Method of claim 1 in which the alkali bisulfite is an alkali metalbisulfite and the alkali borohydride is an alkali metal borohydride.

3. Method of claim 1 in which the alkali bisulfite is sodium bisulfiteand the alkali borohydride is sodium borohydride.

4. Method of claim 1 in which the alkali bisulfite is potassiumbisulfite and the alkali borohydride is potassium borohydride.

5. Method of brightening clay in which the clay is slurried in anaqueous medium, an alkali bisulfite is added to the slurry with stirringin the amount of about 2 to 4.5 pounds per ton of said clay, and alkaliborohydride is added to the resulting slurry in the amount of 0.36 to0.82 pound per ton of said clay, the temperature of the reaction systembeing held at about to 180 F., and the pH being maintained at 3 to 3.5,and the clay is washed free of residual bleaching agents and reactionproducts.

References Cited by the Examiner UNITED STATES PATENTS 1,588,956 6/ 1926Feldenheimer 23l 10 FOREIGN PATENTS 242,358 11/ 1925 Great Britain.

OTHER REFERENCES Sodium Borohydride Potassium Borohydride Manual,published by Metal Hydrides Inc., Beverly, Mass., 1958, page 25.

TOBIAS E. LEVOW, Primary Examiner.

S. MOTT, Assistant Examiner.

1. METHOD OF BRIGHTENING CLAY IN WHICH THE CLAY IS SLURRIED IN ANAQUEOUS MEDIUM, AN ALKALI BISULFITE IS ADDED TO THE SLURRY WITH STIRRINGIN THE AMOUNNT OF ABOUT 0.7 TO 5.6 POUNDS PER TON OF SAID CLAY, AN ALKAIBOROHYDRIDE IS DDED TO THE RESULTING SLURRY IN THE AMOUNT OF 0.125 TO 1POUND PER TON OF SAID CLAY, THE TEMPERATURE OF THE REACTION SYSTEM BEINGHELD AT ABOUT 40* TO 200*F., AND THE PH BRING MAINTAINED AT 2.5 TO 5,AND THE CLAY IS WASHED FREE OF RESIDUAL BLEACHING AGENTS AND REACTIONPRODUCTS.